International Collaboration in Chemistry: Mechanism for Cation Exchange and Doping in Semiconducting Nanostructures

来自加州伯克利大学的菲利普盖斯勒获得了大分子、超分子和纳米化学项目以及化学理论、模型和计算方法项目的国际化学合作奖;他的合作者、特拉维夫大学的Eran Rabani同时得到了美国的支持。以色列两国科学基金会。该项目的动机是，在过去的20年里，化学家在极小尺度上制造材料的能力发生了革命，特别是以“纳米晶体”（纳米尺寸的晶体材料摘录）的形式。然而，实现纳米晶体的巨大技术前景需要对它们的组成、形状、结构和相对排列进行比目前可能的更多的控制。该项目涉及最近发现的一种获得这种控制的途径：将一组离子与一组化学性质不同的离子交换，而不改变离子的结构。这种“阳离子交换”的机制、能量学、动力学和物理后果知之甚少，极大地限制了它的精确应用。该项目使用理论和计算工具来开发和探索模型，揭示离子交换的化学基础。它进一步评估了交换对与理想的技术进步相关的电子特性的影响。该项目的具体活动包括（i）构建用于探索掺杂和离子交换反应机制的模型，（ii）开发和应用探索这些模型的长期行为所需的最先进的计算技术，以及（iii）评估由此产生的电子特性。通过与电子结构计算和实验测量的比较，建立了相互作用模型。采用分子动力学模拟结合过渡路径采样等技术来研究杂质/阳离子如何从溶液中进入晶格，它们如何通过宿主晶格扩散，以及它们如何与宿主原子交换位置。为了计算出出射电子性质，发展了适用于掺杂材料和异质结构的半经验赝势模型。最后，重掺杂纳米晶体的理论在一个“类氢”模型的精神，为多种杂质正在构建一个基本的水平上阐明带的排列，激子的本地化，电子-空穴相互作用，以及这些属性如何取决于大小，形状和材料的选择。

Phillip Geissler from the University of California Berkeley is supported in an International Collaboration in Chemistry award by the Macromolecular, Supramolecular and Nanochemistry program and the Chemical Theory, Models and Computational Methods program; his collaborator, Eran Rabani at Tel Aviv University, is simultaneously supported by the U.S.-Israel Binational Science Foundation. The project is motivated by the fact that the past 20 years have seen a revolution in chemists' ability to make materials on extremely small scales, notably in the form of "nanocrystals" (nanometer-sized excerpts of crystalline materials). Realizing the immense technological promise of nanocrystals, however, requires much more control over their composition, shape, structure, and relative arrangements than is currently possible. This project concerns a recently discovered route towards gaining control of this kind: exchanging one set of ions in a nanocrystal with a chemically distinct set of ions, remarkably without altering the nanocrystal's structure. The mechanism, energetics, dynamics, and physical consequences of such "cation exchange" are very poorly understood, greatly limiting its precise application. The project uses tools of theory and computation to develop and explore models that shed light on the chemical underpinnings of ion exchange. It further evaluates the consequences of exchange on electronic properties relevant to desirable technological advances. Specific activities of this project include (i) construction of models for exploring reaction mechanisms of doping and ion exchange, (ii) development and application of state-of-the-art computational techniques needed to explore the long-time behaviors of these models, and (iii) the assessment of resulting electronic properties. Interaction models are built through judicious comparison with electronic structure calculations and experimental measurements. Molecular dynamics simulations combined with techniques such as transition path sampling are employed to examine how impurities/cations enter the nanocrystal from solution, how they diffuse through the host lattice, and how they exchange places with host atoms. In order to calculate emergent electronic properties, semiempirical pseudopotential models adequate for doped materials and heterostructures are developed. Finally, a theory for heavily doped nanocrystals in the spirit of a "hydrogenic-like" model for multiple impurities is being constructed to elucidate at a fundamental level the alignment of bands, localization of excitons, electron-hole interactions, and how these properties depend on the size, shape, and choice of materials.